Easy

判断一棵二叉树是否对称

Example, 二叉树[1,2,2,3,4,4,3] 对称:
1
/
2 2
/ \ /
3 4 4 3
二叉树[1,2,2,null,3,null,3]不对称
1
/
2 2
\
3 3

方法与相同树类似，相同树是左左比较，右右比较。这里复制自己后，左右比较，右左比较即可。

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def isSymmetric(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        return self.helper(root, root)
            
    def helper(self, x, y):
        if x and y:
            if x.val == y.val:
                return self.helper(x.left, y.right) and self.helper(x.right,y.left)
            else:
                return False
        elif x == y:
            return True
        else:
            return False